1. Field of the Invention
The present invention relates to an image reading apparatus provided with image reading units for a front side and a back side, respectively, and a method for controlling the image reading apparatus.
2. Description of the Related Art
At present, an image reading apparatus that is capable of simultaneously reading the front side and the back side of an original document sheet has been proposed. In the image reading apparatus, two image reading units for the front side and the back side, respectively, are provided in close proximity to each other on an original document conveyance path, and since time periods for which the two image reading units read the original document sheet overlap, scanning speeds (original document conveying speeds) of the two image reading units are set to the same speed during double-sided reading (Japanese Patent Laid-Open No. 2004-040312). The performance of a sensor of the backside image reading unit of this image reading apparatus is lower than the performance of a sensor of the frontside image reading unit, and therefore during double-sided reading, it is necessary to adjust the scanning speed of the frontside image reading unit to conform to the performance of the backside image reading unit. For this reason, the scanning speed of the frontside image reading unit during double-sided reading is reduced so as to be lower than the scanning speed of the frontside image reading unit during single-sided reading.
However, the above-described conventional technology has room for improvement where reading productivity during double-sided reading is concerned. Specifically, in the image reading apparatus according to the above-described conventional technology, when a plurality of original document sheets are read while being conveyed sequentially, the interval between original document sheets is controlled so that the distance between original document sheets, which is the distance between a trailing edge of a preceding original document sheet and a leading edge of a following original document sheet, is a predetermined distance. This distance between original document sheets is always controlled to be constant whether the reading mode is a single-sided reading mode or a double-sided reading mode. Here, according to the technology described in Japanese Patent Laid-Open No. 2004-040312, the image reading unit for the back side that is used at a scanning speed slower than the scanning speed of the image reading unit for the front side is provided, and the scanning speed of the image reading unit for the front side is reduced during double-sided reading. However, if the distance between original document sheets is adjusted to conform to the scanning speed and the image data transfer speed during single-sided reading, during double-sided reading in which the scanning speed is reduced, the original document conveying speed slows down, and the time interval (the time between sheets of paper) between the trailing edge of the preceding original document sheet and the leading edge of the following original document sheet increases accordingly when compared to that during the single-sided reading mode. On the other hand, the image data transfer speed is constant irrespective of the mode, and therefore in the case where the time between sheets of paper is longer than necessary with respect to the image data transfer time, that excess time between sheets of paper is wasted time, resulting in a reduction in the original document reading productivity.
As described above, although the proposal disclosed in Japanese Patent Laid-Open No. 2004-040312 employs a small and low-cost image reading unit for the back side and thus has the advantages that a size reduction and a cost reduction of the image reading apparatus can be realized, a high reading productivity cannot be expected simply by adjusting the scanning speed to conform to the performance of the image reading unit for the back side.